It is common practice to use perforate or slotted tubes in Helmholtz resonator mufflers, as well as in some sections of more complex mufflers, but owing to the tendency for a perforation or slot to cause whistling (noises at about 1000 Hz or more), it has heretofore usually been deemed necessary to have louvres formed in the tubes. This causes secondary difficulties however, in that the louvres are formed outwardly or inwardly or both by lancing and stretching small areas of the tube wall. This operation is usually achieved in a press, the tube either being formed by firstly lancing and subsequent rolling the workpiece, or forming a tube in imperforate form and subsequently lancing the louvres in the tube wall. In the first case the resultant tube has a bead joining its edges, and is usually non-circular and quite unsuitable for accurate fitting to the skirts on the ends of a muffler housing or accurate fitting to a properly rounded tube. Furthermore, in both cases, outstanding louvre edges prevent the tube from being driven through a preformed muffler housing or preformed internal baffle, and inwardly facing louvre edges prevent a close-fitting tube being placed inside the louvred tube.
The main object of this invention is to provide improvements whereby a perforate or slotted tube can be employed, without the need for louvres.